Published on October 13th 2017 in

Damage on aluminium and composite may undetectable to the naked eye

Potential growth in carbon fibre and aluminium use among OEMs could lead to shops having to be cognisant of the potential for damage at the microscopic level.

“While extrusions and cast parts are likely to be replace-only, it is possible to repair aluminium panels”, P&L Consultants co-owner Larry Montanez said on the ‘Repair University Live’. “A fracture in cast aluminium creates microfractures, and welding that area generates ‘tiny explosions’ and more microfractures on the part. Even an aluminium sheet panel is prone to ‘microcracking’, ‘ripping’ and ‘tearing’ and a shop might think it can repair such a torn part, but it’s not going to last” he added.

“Heat from welding a damaged panel creates cracking on the outside of the panel and most outer panels are 0.9 mm thick, but if it’s torn or ripped, the damaged area dwindles to 0.6-0.7 mm”, according to Montanez. “When you try to weld and fill the void, the action causes heat cracking, thinning and work hardening. If you have a crack, rip or tear on the edge of a part like a guard, it’s probably going to break again following such a repair”, Montanez said.

However, aluminium dent repair often requires a little heat, “Repair University” host Kristen Felder observed. However, low heat only, as aluminium melts at less than half the temperature steel does and is compromised long before that melting point. When applying that heat, repairers need to think at the microscopic level, according to Montanez. Never use propane torches to generate that heat, for it adds moisture to the expanded aluminium molecules, Montanez said. And never quick-cool the metal, or you’ll create microfractures. “You gotta let it cool down itself, even though this could add time to the repair process”’ he said.

“If the paint on the part has cracked from a rapid drop in temperature, it’s a good bet microcracking has also occurred”, Montanez said. “If you can see the paint failures – what can’t you see?” he added. He argued that every shop performing structural aluminium work should have a microfluxing dye kit to test for microfractures, particularly for welding. He gave the example of a Porsche Panamera 970, which has a piece of cast aluminium attached to extruded aluminium front rails. Porsche demands a shop dye-test the part for microfractures, which if present could compromise crashworthiness for a future collision.

Carbon fibre, which might be further into the future but sooner than we expect, can also be compromised at the microscopic level while appearing flawless to the naked eye, General Motors body structures advanced composites engineering group manager Mark Voss said last year at SEMA 2016.

In fact, unlike aluminium or steel panels, which deform when impacted with “nice, visible evidence,” a composite hood might not give any indication it’s been dented, according to Voss. “General Motors once hit a Corvette C6 with a pedestrian dummy in a test. While the interior of the hood was ‘completely destroyed’ … the hood on the outside appeared good as new,” he said.

In another test, a carbon fibre hood was run into a barrier at 35 mph. It popped back into place, the damage largely invisible to the eye. Tools like a micrograph are necessary to see that something’s wrong, he said. (Which could make photo estimating even more difficult should OEMs replace one or two panels with composites.)

“There is ‘no easy answer’ on how to tell if a composite part was undamaged”, Voss said. “Before composites can truly go mass-market, GM would need to figure out a way for auto body technicians to do so”, he added. However, Voss noted that much “exciting technology” could be used for this purpose, citing dyes as an example.

This article courtesy of John Huetter of Repairer Driven Education (RDE). Check out their website at; http://www.repairerdrivennews.com/ for this and many other informative and educational articles on the collision repair industry.

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